The invention is directed to rigid polyurethane foams and methods for making rigid polyurethane foams.
Polyurethane foams are formed by the reaction of a polyisocyanate compound, such as toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI) with a polyhydroxyl compound, such as a polyol. Generally, streams of equal volume of the polyol (i.e., polyol side) and polyisocyanate (isocyanate) are intermixed in a mixing head and then injected into a mold where they react to form the polyurethane foam. Generally, the polyol side also contains water, surfactant, catalysts and added blowing agents.
Generally, there are two types of polyurethane foams: flexible and rigid. In general, flexible foams have open cellular structures and a flexible polyurethane (e.g., uses a low functionality; high molecular weight polyol) which allows them to be elastically deformed. Generally, when making a flexible polyurethane foam, water in the polyol side is used as the blowing agent. The water reacts with the isocyanate producing carbon dioxide that foams the polyurethane as the isocyanate and polyol react.
Rigid foams, on the other hand, generally have a substantially closed cellular structure which essentially fails to elastically deform (i.e., when a rigid foam deforms, it deforms permanently). To provide rigidity, rigid polyurethane foams, typically, are formed using a lower molecular weight polyol than used to make a flexible foam and also a cross-linking polyol. Generally, the cross-linking polyol has (1) a hydroxyl functionality of greater than 3 to 8 (i.e., typically greater than 3 to 8 hydroxyl groups/molecule that can react with the isocyanate), (2) a mean molecular weight of 300 to 800 and high viscosity of 3000 to 20,000 centipoise. The cross-linking polyols are typically added to increase the cross-linking density to form a rigid foam of adequate strength and rigidity.
Unfortunately, the use of high viscosity cross-linking polyols generally raises the viscosity of the polyol side substantially. The increased viscosity of the polyol side typically makes it difficult to achieve efficient mixing with the low viscosity isocyanate side, resulting in inhomogeneous rigid foams. Historically, low viscosity, liquid volatile organic compounds (i.e., added liquid blowing agents) have been used to lower the viscosity. However, this results in volatile organic compound (VOC) emissions when making the foam.
The cross-linking polyols also make it difficult to balance the volumes of the isocyanate side and polyol side due to the high equivalent weight of the cross-linking polyol. This is especially true when the polyol side contains water due to its low equivalent weight of 9. Again, the aforementioned volatile organic compounds are generally added to balance the volume of the polyol and isocyanate side and to blow the foam in the absence of water.
In addition, the cross-linking polyols cause the foam to achieve xe2x80x9cgel pointxe2x80x9d sooner than a foam formed without them. Gel point is when the viscosity of the foaming mass begins to rise exponentially due to link up of polymer domains. Thus, rigid foams made with cross-linking polyols tend to split when made with water because of internal gas pressure from the continued evolution of CO2 after the foam has gelled.
Consequently, the blowing agent for a rigid foam generally is either (1) a liquid volatile organic compound, such as chloromethane (e.g., CFM-11), that volatizes during the forming of the polyurethane causing the polyurethane to foam or (2) an gaseous organic compound, such as chloromethane (e.g., CFM-12), that is injected into the streams causing the streams to froth and consequently form the rigid foam. These blowing agents have generally been used to avoid one or more of the problems described above. However, they raise environmental and safety concerns.
Thus, it would be desirable to provide a rigid polyurethane foam that avoids one or more of the problems of the prior art, such as one or more of those described above.
A first aspect of the present invention is a method for forming a polyurethane foam comprising: contacting a first reactant comprised of a polyisocyanate having an average isocyanate functionality of at least 2 and a second reactant comprised of a low molecular weight compound that has at least two to, at most, three groups containing an active hydrogen in the presence of water for a time sufficient to form a substantially rigid foam, provided the foam is formed essentially in the absence of a cross-linking polyol.
A second aspect of the invention is a polyurethane foam comprising the reaction product of a first reactant comprised of a polyisocyanate having an average isocyanate functionality of at least 2 and a second reactant comprised of a low molecular weight compound that has at least two to, at most, three groups containing an active hydrogen and water, wherein the reaction product is formed essentially in the absence of a cross-linking polyol and the polyurethane foam is substantially rigid. A substantially rigid foam, herein, is a rigid foam as understood in the art. For example, the substantially rigid foam generally has a closed cellular structure which essentially fails to elastically deform (i.e., any deformation of the foam tends to be permanent).
Herein, the cross-linking polyol has a hydroxyl functionality of greater than 3 (i.e., greater than 3 hydroxyl groups/molecule that can react with the isocyanate) and a molecular weight of about 300 to 800. Generally, the cross-linking polyol has a viscosity of 3000 to 20,000 centipoise. The foam formed essentially in the absence of the cross-linking polyol means that only trace amounts are present in the reaction mixture that forms the foam. Preferably there is no cross-linking polyol.
By using a low molecular weight compound, such as propylene glycol, a substantially rigid polyurethane foam may surprisingly be formed in the absence of a cross-linking polyol. The foam may also be formed in the absence of a blowing agent other than CO2 produced from the water-polyisocyanate reaction. It is believed that the low molecular weight compound slows the cross-linking and, consequently, the onset of rigidity of the foam being formed. This slowing is thought to provide a sufficient time for essentially complete evolution of CO2 from the water isocyanate reaction to allow the foam to form without splitting, as occurs, for example, when using the cross-linking polyol described above. In addition, it is also believed that the use of the low molecular weight compound more completely reacts with the isocyanate groups, resulting in foams generally having higher compressive moduli than those made with cross-linking polyols.
In addition, because of the low equivalent weight of the low molecular weight compound, the first aspect of the invention may also be advantageously performed using volumes of the first and second reactants that are similar, even when the second reactant contains an auxiliary polyol, such as a polyether polyol described later while maintaining the isocyanate index near one. Consequently, the method of the first aspect may be performed using standard polyurethane process equipment. The use of the low molecular weight compound having a low viscosity also results in the second reactant (i.e., the polyol side) to have a low viscosity similar to known polyisocyanates. The viscosity similarity allows the two reactants to be easily mixed and reacted to form a more uniform and homogeneous foam.
The method and foams produced according to the present invention may be used in any suitable application, such as those known in the art, including applications involving, for example, automotive applications requiring stiffening, reinforcing, NVH (noise, vibration and harshness) abatement in a vehicle.
The method according to this invention contacts a first reactant comprised of a polyisocyanate having a functionality of at least 2 and a second reactant, comprised of a low molecular weight compound, that has at least two to, at most, three groups containing an active hydrogen in the presence of water.
The polyisocyanate may be any polyisocyante suitable for making a polyurethane foam, such as those known in the art. The polyisocyanate may be an aromatic or aliphatic polyisocyanate, polymeric isocyanate, aromatic diisocyanate and aliphatic diisocyanate. Exemplary polyiisocyanates include m-phenylene diisocyanate, tolylene-2-4-diisocyanate, tolylene-2-6-diisocyanate, hexamethylene-1,6-diisocyanate, tetramethylene-1,4-diisocyanate, cyclohexane-1,4-diisocyanate, hexahydrotolylene diisocyanate, naphthylene-1,5-diisocyanate, methoxyphenyl-2,4-diisocyanate, diphenylmethane-4,4,-diisocyanate, 4,4xe2x80x2 biphenylene diisocyanate, 3,3xe2x80x2 dimethoxy-4,4xe2x80x2-biphenyl diisocyanate, 3,3xe2x80x2-dimethyl-4-4xe2x80x2-biphenyl diisocyanate, 3,3xe2x80x2-dimethyldiphenylmethane-4,4xe2x80x2-diisocyanate, 4,4xe2x80x2,4xe2x80x3-triphenyl methane triisocyanate, polymethylene polyphenylisocyanate and tolylene-2,4,6-triisocyanate, 4,4xe2x80x2-dimethyldiphenylmethane-2,2xe2x80x25,5xe2x80x2-tetraisocyanate. Preferably the polyisocyanate is diphenylmethane-4,4xe2x80x2-diisocyanate (MDI), tolylene-2-4-diisocyanate, tolylene-2-6-diisocyanate or mixtures thereof. Tolylene-2-4-diisocyanate, tolylene-2-6-diisocyanate and mixtures thereof are generically referred to as TDI. More preferably, the polyisocyanate is a polymeric polyisocyanate formed from MDI, such as those available from The Dow Chemical Company under the PAPI(trademark) trade name. The polymeric polyisocyanate xe2x80x9cPAPI 27xe2x80x9d is particularly preferred.
Generally, the average isocyanate functionality of the polyisocyanate is at least 2 to at most about 6. Preferably the average isocyanate functionality of the polyisocyanate is at least about 2.5 and more preferably at least about 2.7 to preferably at most about 3.5 and more preferably at most about 3.3. As understood in the art, functionality is the average number of isocyanate groups per molecule in the polyisocyanate.
To ensure adequate cross-linking, the low molecular weight compound (LMWC) has a functionality of at least 2 to at most 3, where the functionality is the number of hydroxyl or equivalent hydrogen (e.g., amine) reactive sites per molecule (i.e., the compound has at least two groups containing an active hydrogen). Generally, the groups of the LMWC are an amine, thiol or hydroxyl. The LMWC may be, for example, a diol, dithiol, hydroxy-amine, hydroxy-thiol, amino-thiol or a diamine. The LMWC may be aliphatic or aromatic, aliphatic being preferred. It is preferred that at least one of the groups is a primary group and at least one other group is a secondary group. For example, propylene glycol has one primary hydroxyl and one secondary hydroxyl. The presence of a secondary group is believed to slow the reaction with the isocyanate and, consequently, results in a foam that is easier to form without splitting. The groups of the LMWC are preferably hydroxyl groups. Exemplary LMWCs include propylene glycol, ethylene glycol, 1-4 butanediol, 1-6 hexanediol, resorcinal, hydroquinone, monoethanolamine, glycerin, trimethylolpropane, diethanolamine, triethanolamine, diethylene glycol, dipropylene glycol, neopentyl glycol, hydroquinone bis(2-hydroxyethyl) ether or mixtures thereof. Preferably the LWMC is propylene glycol, ethylene glycol or glycerine. More preferably the LMWC is propylene glycol.
Surprisingly, a substantially rigid foam and adequate cross-linking may be formed when a LMWC having a functionality of less than 3 (e.g., 2) is used in conjunction with a polyisocyanate having a functionality of greater than 2. It is surprising since cross-linking polyols are understood in the art to be compounds having a functionality of more than 3.
The low molecular weight compound must also have a sufficiently low molecular weight to form a substantially rigid polyurethane foam. If the molecular weight is too high, a substantially rigid foam is not formed. Generally, the molecular weight of the LMWC is at most about 200, preferably at most about 150, more preferably at most about 100 to preferably at least about 45.
The amount of LMWC is also important in the formation of a rigid foam. If the amount is insufficient, the foam that is formed may not be rigid. Generally, the amount of LWMC is at least about 2.5 percent by weight of the polyurethane reaction mixture (i.e., all of the components used to make the foam). Preferably the amount of the LWMC is at least about 3 percent, more preferably at least about 5 percent, even more preferably at least about 7.5 percent and most preferably at least about 10 percent by weight of the polyurethane reaction mixture (i.e., all of the components used to make the foam). Generally, these amounts of LMWC correspond to the LWMC comprising at least about 2.5 percent, preferably at least about 6 percent, more preferably at least about 10 percent and most preferably at least about 15 percent by weight of the second reactant.
Even though the second reactant may be entirely composed of the LMWC, it is preferred that the amount is less than 50 percent by weight of the second reactant so that the volume of the first reactant and second reactant may be similar as described herein. Consequently, the second reactant may also contain an auxiliary polyol in addition to the LMWC. Herein, the auxiliary polyol may be a polyol, such as those described by U.S. Pat. Nos. 3,383,351; 3,823,201; 4,119,586 and 4,148,840, each incorporated herein by reference. Exemplary auxiliary polyols include polyhydroxyalkane polyols, polytetrahydrofuran polyols, polyoxyalkylene polyols, alkylene oxide adducts of polyhydroxyalkanes, alkylene oxide adducts of non-reducing sugars and sugar derivitives, alkylene oxide adducts of phosphorus and polyphosphorus acids, alkylene oxide adducts of polyphenols and polyols derived from natural oils, such as caster oil. Preferably the polyols are glycols, triols or higher functionality polyols of poly(oxybutylene), poly(oxyethylene), poly(oxypropylene), poly(oxypropylene-oxyethylene) or mixtures thereof. Generally, these polyols have a molecular weight of at least about 300 MW. The auxiliary polyols used in the instant invention are understood to be incapable of forming a substantially rigid foam in the absence of the LMWC (i.e., they are not cross-linking polyols as described herein). For example, the auxiliary polyol may have an average functionality of greater than 2, but the chain length of the auxiliary polyol is of a length and functionality that fails to cause an amount of cross-linking sufficient to make a substantially rigid foam.
The auxiliary polyol may have a hydroxyl number that varies over a large range depending upon the desired polyurethane foam properties. In general, the auxiliary polyol may have a hydroxyl number that ranges from about 20 to about 1000. Preferably the hydroxyl number is at least about 25, and more preferably at least about 30 to preferably at most about 600, and more preferably at most about 450. The hydroxyl number is defined as the number of milligrams of potassium hydroxide required for the complete hydrolysis of the fully acetylated derivative prepared from 1 gram of polyol.
The method may also be carried out in the presence of catalysts, such as those described by U.S. Pat. No. 4,390,645, at col. 10, lines 14 to 27; surface active agents, such as those described by U.S. Pat. No. 4,390,645, at col. 10, lines 28 to 43; chain extending agents, such as those described by U.S. Pat. No. 4,390,645, at col. 10, lines 59 to 68, and col. 10, lines 1 to 5; fillers, such as calcium carbonate and pigments, such as titanium dioxide, iron oxide, chromium oxide, azo/diazo dyes, phthalocyanines, dioxazines and carbon black, the above relevant portions of U.S. Pat. No. 4,390,645 being incorporated herein by reference. The method may also be carried out in the presence of a flame retardant, such as those known in the art, and may include, for example, phosphoruous compounds, halogen containing compounds and melamine.
More specifically, representative catalysts include:
(a) tertiary amines, such as trimethylamine, triethylamine, N-N-methylmorpholine, N-ethylmorpholine, N,N-dimethylbenzylamine, N,N-dimethylethanolamine, N,N,Nxe2x80x2,Nxe2x80x2-tetramethyl-1,4-butanediamine, N,N-dimethylpiperazine, 1,4-diazobicyclo[2,2,2]octane, bis(dimethylaminoethyl)ether and triethylenediamine;
(b) tertiary phosphines, such as trialkylphosphines and dialkylbenzylphosphines;
(c) chelates of various metals, such as those which can be obtained from acetylacetone, benzoylacetone, trifluoroacetyl acetone, ethyl acetoacetate and the like with metals, such as Be, Mg, Zn, Cd, Pd, Ti, Zr, Sn, As, Bi, Cr, Mo, Mn, Fe, Co and Ni;
(d) acidic metal salts of strong acids, such as ferric chloride, stannic chloride, stannous chloride, antimony trichloride, bismuth nitrate and bismuth chloride;
(e) strong bases, such as alkali and alkaline earth metal hydroxides, alkoxides and phenoxides;
(f) alcoholates and phenolates of various metals, such as Ti(OR)4, Sn(OR)4 and Al(OR)9, wherein R is alkyl or aryl and the reaction products of the alcoholates with carboxylic acids, beta-diketones and 2-(N,N-dialkylamino)alcohols;
(g) salts of organic acids with a variety of metals, such as alkali metals, alkaline earth metals, Al, Sn, Pb, Mn, Co, Ni and Cu including, for example, sodium acetate, stannous octoate, stannous oleate, lead octoate, metallic driers, such as manganese and cobalt naphthenate;
(h) organometallic derivatives of tetravalent tin, trivalent and pentavalent As, Sb and Bi and metal carbonyls of iron and cobalt and
(i) mixtures thereof. Catalysts are typically used in small amounts, for example, each catalyst being employed from about 0.0015 to about 1 percent by weight of the polyurethane reaction mixture (i.e., all of the components used to make the foam).
Particular examples of surface active agents include nonionic surfactants and wetting agents, such as those prepared by the sequential addition of propylene oxide and then ethylene oxide to propylene glycol, the solid or liquid organosilicones, polyethylene glycol ethers of long chain alcohols, tertiary amine or alkylolamine salt of long chain alkyl acid sulfate esters, alkyl sulfonic ester and alkyl arylsulfonic acids. The surface active agents prepared by the sequential addition of propylene oxide and then ethylene oxide to propylene glycol and the solid or liquid organosilicones are preferred. Liquid organosilicones which are not hydrolyzable are more preferred. Examples of non-hydrolyzable organosilicones include those available under the trademarks xe2x80x9cDABCO(trademark) DC 5043,xe2x80x9d xe2x80x9cDABCO(trademark) DC 5169xe2x80x9d and xe2x80x9cDABCO(trademark) DC 5244,xe2x80x9d available from Dow Corning Corp., Freeland, Mich. and xe2x80x9cTEGOSTAB(trademark) B-8404 and TEGOSTAB(trademark) 8462, available from Th. Goldschmidt Chemical Corp., Hopewell, Va. Surface active agents are typically used in small amounts, for example, from about 0.0015 to about 1 percent by weight of the polyurethane reaction mixture (i.e., all of the components used to make the foam).
When forming the foam, it is preferred that the only blowing agent is essentially the CO2 produced by the water isocyanate reaction. Another blowing agent may be present, such as a low boiling hydrocarbon, such as pentane, hexane, heptane, pentene, and heptene, directly added carbon dioxide, an azo compound, such as azohexahydrobenzodnitrile or a halogenated hydrocarbon, such as dichlorodifluoroethane, vinylidene chloride and methylene chloride. Generally, the amount of these blowing agents is small. Preferably the amount of these blowing agents is at most a trace amount and more preferably none at all (i.e., the only blowing agent is CO2 generated in situ from the water-isocyanate reaction).
The foam may be made by any suitable method, such as those known in the art. The method may include, for example, prepolymer (described in U.S. Pat. No. 4,390,645), one shot (described in U.S. Pat. No. 2,866,744) or frothing (described in U.S. Pat. Nos. 3,755,212; 3,849,156 and 3,821,130), each of the aforementioned patents is incorporated herein by reference.
The first reactant and second reactant are contacted for a time sufficient to form the substantially rigid polyurethane foam without splitting. Generally, the time is as short as practicable and may be from about 1 second to about 60 minutes. The temperature of the reaction may be any sufficient to form the foam without splitting but should not be so great that the polyurethane foam decomposes. Generally, the temperature ranges from room temperature up to about 200xc2x0 C.
It is preferred, when forming the foam, that the volume of the first reactant and second reactant are similar so that typical polyurethane foaming apparatus may be used. Generally, the volume ratio of the first reactant to the second reactant is at least about 0.7, more preferably at least about 0.8 and most preferably at least about 0.9 to preferably at most about 1.3, more preferably at most about 1.2 and most preferably at most about 1.1. The second reactant, besides containing the LMWC and polyol, may contain, for example, a catalyst, filler, water, flame retardant and surfactant.
Because LMWCs generally have a low viscosity, the present invention enables a more uniform mixing of the first and second reactant than the prior art. Improved mixing provides a more uniform (i.e., more consistent cell size and structure) and homogeneous foam. The viscosity of the second reactant containing the LMWC generally has a viscosity that is within 0.5 to 1.5 times the viscosity of the first reactant (i.e., the polyisocyanate used). Preferably the viscosity of the second reactant is at least about 0.7, more preferably at least about 0.8, and most preferably at least about 0.9 to preferably at most about 1.3, more preferably at most about 1.2 and most preferably at most about 1.1 times the viscosity of the first reactant (i.e., the polyisocyanate).
In the absence of an inert diluent, the apparent visocosity of the second reactant preferably is within a range of about 50 to about 300 centipoise. More preferably the viscosity is at most about 250 centipoise and most preferably at most about 200 centipoise in the absence of an inert diluent. Herein, an inert diluent is a liquid that lowers the viscosity of the second reactant but fails to affect the urethane reaction or react with either hydroxyl or isocyanate groups. Examples of inert diluents may include blowing agents, such as CFCs or plasticizers, such as phthalates.
When forming the foam, the amount of polyisocyanate and, consequently, other reactants used in making polyurethane is commonly given by the isocyanate index. The isocyanate index can be given by the equation:       Isocyanate Index    =            Actual amount of isocyanate used              Theoretical amount of isocyanate      
The theoretical equivalent amount of isocyanate is the stoichiometric amount of isocyanate required to react with the polyol and any other reactive additives, such as water. The isocyanate index may vary over a wide range, depending on the foam characteristics desired. Generally, a higher index produces a harder foam. In the production of the rigid foams of this invention, the isocyanate index typically ranges from about 0.7 to 1.4. Preferably the index is at least about 0.75, more preferably at least about 0.8, even more preferably at least about 0.85 and most preferably at least about 0.9 to preferably at most about 1.35, more preferably at most about 1.3, even more preferably at most about 1.25 and most preferably at most about 1.2. Large excess in the isocyanate may be used, if it is desired, to make, for example, an isocyanurate foam.
The substantially rigid foam that is formed may have a large range of properties, depending on the particular application that is desired. For example, the foam may have a bulk density of about 5 to about 50 pounds per cubic foot. The foam may also have a wide range of compressive strengths, depending, of course, on the density and particular components used. For example, the foam may have a compressive strength of about 100 to about 5000 pounds per square inch and a compressive modulus of about 2000 to about 100,000 pounds per square inch.